'use strict';

/* eslint-disable no-var */

var reusify = require('reusify');

function fastqueue(context, worker, _concurrency) {
  if (typeof context === 'function') {
    _concurrency = worker;
    worker = context;
    context = null;
  }

  if (!(_concurrency >= 1)) {
    throw new Error('fastqueue concurrency must be equal to or greater than 1');
  }

  var cache = reusify(Task);
  var queueHead = null;
  var queueTail = null;
  var _running = 0;
  var errorHandler = null;

  var self = {
    push: push,
    drain: noop,
    saturated: noop,
    pause: pause,
    paused: false,

    get concurrency() {
      return _concurrency;
    },
    set concurrency(value) {
      if (!(value >= 1)) {
        throw new Error(
          'fastqueue concurrency must be equal to or greater than 1'
        );
      }
      _concurrency = value;

      if (self.paused) return;
      for (; queueHead && _running < _concurrency; ) {
        _running++;
        release();
      }
    },

    running: running,
    resume: resume,
    idle: idle,
    length: length,
    getQueue: getQueue,
    unshift: unshift,
    empty: noop,
    kill: kill,
    killAndDrain: killAndDrain,
    error: error,
  };

  return self;

  function running() {
    return _running;
  }

  function pause() {
    self.paused = true;
  }

  function length() {
    var current = queueHead;
    var counter = 0;

    while (current) {
      current = current.next;
      counter++;
    }

    return counter;
  }

  function getQueue() {
    var current = queueHead;
    var tasks = [];

    while (current) {
      tasks.push(current.value);
      current = current.next;
    }

    return tasks;
  }

  function resume() {
    if (!self.paused) return;
    self.paused = false;
    if (queueHead === null) {
      _running++;
      release();
      return;
    }
    for (; queueHead && _running < _concurrency; ) {
      _running++;
      release();
    }
  }

  function idle() {
    return _running === 0 && self.length() === 0;
  }

  function push(value, done) {
    var current = cache.get();

    current.context = context;
    current.release = release;
    current.value = value;
    current.callback = done || noop;
    current.errorHandler = errorHandler;

    if (_running >= _concurrency || self.paused) {
      if (queueTail) {
        queueTail.next = current;
        queueTail = current;
      } else {
        queueHead = current;
        queueTail = current;
        self.saturated();
      }
    } else {
      _running++;
      worker.call(context, current.value, current.worked);
    }
  }

  function unshift(value, done) {
    var current = cache.get();

    current.context = context;
    current.release = release;
    current.value = value;
    current.callback = done || noop;
    current.errorHandler = errorHandler;

    if (_running >= _concurrency || self.paused) {
      if (queueHead) {
        current.next = queueHead;
        queueHead = current;
      } else {
        queueHead = current;
        queueTail = current;
        self.saturated();
      }
    } else {
      _running++;
      worker.call(context, current.value, current.worked);
    }
  }

  function release(holder) {
    if (holder) {
      cache.release(holder);
    }
    var next = queueHead;
    if (next && _running <= _concurrency) {
      if (!self.paused) {
        if (queueTail === queueHead) {
          queueTail = null;
        }
        queueHead = next.next;
        next.next = null;
        worker.call(context, next.value, next.worked);
        if (queueTail === null) {
          self.empty();
        }
      } else {
        _running--;
      }
    } else if (--_running === 0) {
      self.drain();
    }
  }

  function kill() {
    queueHead = null;
    queueTail = null;
    self.drain = noop;
  }

  function killAndDrain() {
    queueHead = null;
    queueTail = null;
    self.drain();
    self.drain = noop;
  }

  function error(handler) {
    errorHandler = handler;
  }
}

function noop() {}

function Task() {
  this.value = null;
  this.callback = noop;
  this.next = null;
  this.release = noop;
  this.context = null;
  this.errorHandler = null;

  var self = this;

  this.worked = function worked(err, result) {
    var callback = self.callback;
    var errorHandler = self.errorHandler;
    var val = self.value;
    self.value = null;
    self.callback = noop;
    if (self.errorHandler) {
      errorHandler(err, val);
    }
    callback.call(self.context, err, result);
    self.release(self);
  };
}

function queueAsPromised(context, worker, _concurrency) {
  if (typeof context === 'function') {
    _concurrency = worker;
    worker = context;
    context = null;
  }

  function asyncWrapper(arg, cb) {
    worker.call(this, arg).then(function (res) {
      cb(null, res);
    }, cb);
  }

  var queue = fastqueue(context, asyncWrapper, _concurrency);

  var pushCb = queue.push;
  var unshiftCb = queue.unshift;

  queue.push = push;
  queue.unshift = unshift;
  queue.drained = drained;

  return queue;

  function push(value) {
    var p = new Promise(function (resolve, reject) {
      pushCb(value, function (err, result) {
        if (err) {
          reject(err);
          return;
        }
        resolve(result);
      });
    });

    // Let's fork the promise chain to
    // make the error bubble up to the user but
    // not lead to a unhandledRejection
    p.catch(noop);

    return p;
  }

  function unshift(value) {
    var p = new Promise(function (resolve, reject) {
      unshiftCb(value, function (err, result) {
        if (err) {
          reject(err);
          return;
        }
        resolve(result);
      });
    });

    // Let's fork the promise chain to
    // make the error bubble up to the user but
    // not lead to a unhandledRejection
    p.catch(noop);

    return p;
  }

  function drained() {
    var p = new Promise(function (resolve) {
      process.nextTick(function () {
        if (queue.idle()) {
          resolve();
        } else {
          var previousDrain = queue.drain;
          queue.drain = function () {
            if (typeof previousDrain === 'function') previousDrain();
            resolve();
            queue.drain = previousDrain;
          };
        }
      });
    });

    return p;
  }
}

module.exports = fastqueue;
module.exports.promise = queueAsPromised;